Studies on mechanical and wear properties of alloyed hypereutectic gray cast irons in the as-cast pearlitic and austempered conditions

The mechanical and wear behavior of a series of as-cast gray iron alloys were compared with properties obtained after austempering at 360 °C. The austempered alloys showed equivalent or moderately enhanced mechanical strength than the as-cast pearlitic gray irons. The specific wear rates of all the austempered alloys decreased significantly by 7–15 times and friction coefficient reduced by 30–50% compared to pearlitic alloyed gray irons. The dry sliding wear studies of as-cast alloys against high carbon 1%Cr through-thickness hardened steel shows that the specific wear rate ranged from 5.6 to 19.1 (×10⁻⁷) g/Nm with friction coefficient from 0.55 to 0.7. While, the austempered alloys showed wear rates from 0.5 to 2.6 (×10⁻⁷) g/Nm with friction coefficient ranging from 0.23 to 0.4. The improved wear resistance was attributed to the layer wise surface phase transformation associated with strain induced martensite formation of the stabilized austenite in the austempered matrix, lubrication of the interface by the flake graphite and better heat conduction from the rubbing interface by higher volume fraction of the graphite. Cast iron alloyed with Ni shows enhanced mechanical properties and wear resistance. The tensile strength shows decreasing trend with increase in carbon equivalent and graphite volume. The specific wear rate and friction coefficient shows decreasing trend with increase in hardness and graphite flake volume.     

Microstructure and mechanical properties of Hadfield steel matrix composite reinforced with oriented high-chromium cast iron bars

To obtain a compatible material of high hardness and high toughness, Hadfield steel matrix was reinforced by oriented high-chromium cast iron bars. The mechanical behaviors of the as-cast and water-quenched composites were comparatively studied with a Hadfield steel substrate. The experimental results showed that the alloy powders inside the flux-cored welding wires could be melted by the heat capacity of Hadfield steel melt and became high-chromium cast iron bars. The impact toughness of the water-quenched composite was higher than that of the as-cast composite and lower than that of the Hadfield steel. The wear rate of the water-quenched composite was 1.23 mg/h m² at 0.3 kg and 2.93 mg/h m² at 1.2 kg, which was lower compared with those of the as-cast composite and Hadfield steel. The impact toughness and wear resistance of the water-quenched composite were related not only to the combining actions of the Hadfield steel matrix and high-chromium cast iron bars but also to the effect of heat treatment. The wear behavior of the water-quenched composite was industrially tested as pulverizer plate.     

Dry sliding wear behaviour of Al–12Si–4Mg alloy with cerium addition

The purpose of this work is to understand the effect of cerium addition on wear resistance behaviour of as-cast alloys. Al–12Si–4 Mg alloys with 1–5 wt% cerium addition were prepared using the casting technique. A sliding wear test was carried out under applied loads of 10 N, 30 N and 50 N at a fixed sliding speed of 1 m/s using a pin-on-disc configuration. The wear test was conducted in dry conditions at room temperature of 25 °C. Detailed analysis of the microstructure, worn surface, collected debris and microhardness was undertaken in order to investigate the differences between the as-cast alloys with different levels of cerium addition. The addition of 1–5 wt% cerium was found to lead to the precipitation of intermetallic phases (Al–Ce), resulting a needle-like structures. Increasing cerium content up to 2 wt% improved both wear resistance and microhardness of as-cast alloys. Addition of more than 2 wt% cerium, however, led to a decrease in microhardness, resulting in lower wear resistance of the alloys. Moderate wear was observed at all loads, with specific wear rates (K′) ranging from 6.82 × 10⁻⁵ with 2 wt% Ce at applied load of 50 N to 21.48 × 10⁻⁵ mm³/N m without added Ce at an applied load of 10 N. Based on K′ ranges, the as-cast alloys exhibited moderate wear regimes, and the mechanism of wear is a combination of abrasion and adhesion. Alloy containing 2 wt% Ce, with the highest hardness and lowest K′ value, showed the greatest wear resistance.     

Effect of misch metal inoculation on microstructure, mechanical and wear properties of hypoeutectic gray cast irons

Alloyed gray cast irons were made with and without misch metal inoculation (0.1%). The mechanical and wear properties were compared with conventional gray cast iron used for a typical clutch application in heavy commercial vehicles. Alloyed gray iron without misch metal showed higher volume fraction of pearlite (89%) and lower flake graphite (11%). Misch metal inoculated gray irons showed higher volume of flake graphite (15%) with 85% pearlite as matrix. Alloyed gray irons produced tensile strength from 300 to 344 MPa and hardness in the range of 221–247 VHN. Misch metal inoculation has slightly increased the graphite volume (40–60%) with corresponding decrease in strength and hardness (9–13%) in alloyed gray irons. The specific wear rates of all alloyed gray irons are significantly lower (<34%) compared to unalloyed base at two different sliding speeds (1.6 m/s and 2.5 m/s). The friction coefficient is less than 0.4 for alloyed gray irons as against 0.5–0.7 for unalloyed base gray iron at both the sliding speeds. This is attributed to the presence of alloying additives within the matrix which resists adhesive and abrasive wear loss. Among the inoculated alloyed gray irons, the alloy with lower S content (0.08%) showed higher wear rate at higher sliding speed due to lower graphite flake density compared to higher S containing iron (0.12%). This indicates that the wear rate is influenced by the amount of graphite which is released into the interface during sliding to provide lubrication and reduce wear. Inoculation with rare earth misch metal has a positive influence over graphite morphology in gray iron.     

Effect of loading rate on the monotonic tensile behavior and tensile strength of an oxide–oxide ceramic composite at 1200 °C

The influence of loading rate on monotonic tensile behavior and tensile properties of an oxide–oxide ceramic composite was evaluated in laboratory air at 1200 °C. The composite consists of a porous alumina matrix reinforced with woven mullite/alumina (Nextel™720) fibers, has no interface between the fiber and matrix, and relies on the porous matrix for flaw tolerance. Tensile tests conducted at loading rates of 0.0025 and 25 MPa/s revealed a strong effect of rate on the stress–strain behavior as well as on the ultimate tensile strength (UTS), elastic modulus and failure strain. At 0.0025 MPa/s, increase in stress results in non-monotonic change in strain, with the rate of change of strain reversing its sign at stresses 25 MPa/s. Several samples were subjected to additional heat treatments prior to testing in order to determine whether this unusual stress–strain behavior was an artifact of incomplete processing of fibers in the as-received material. The unusual material response in the 0–30 MPa stress range was further investigated in creep tests conducted with the applied stresses ≤26 MPa. Negative creep (i.e. decrease in strain under constant stress) was observed. Porosity measurements indicate that a decrease in matrix porosity and matrix densification may be taking place in the N720/A composite exposed to 1200 °C at stresses <30 MPa for prolonged periods of time.     

(Fe,Cr)7C3/Fe surface gradient composite: Microstructure,microhardness, and wear resistance

In this study, the (Fe,Cr)₇C₃/Fe surface gradient composite was produced by in situ synthesis process with subsequent heat treatment. According to the results of thermal analysis, the as-cast specimen was subjected to heat treatment at 1180 °C for 3 h in argon atmosphere. The phase composition, microstructure, microhardness, and wear resistance of the samples were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), Vickers hardness tester, and wear resistance tester, respectively. The XRD results show that α-Fe and (Fe,Cr)₇C₃ are the predominant crystalline phases in the composite obtained. The volume fraction of (Fe,Cr)₇C₃ particulates has a gradient distribution from the surface to the matrix, and the morphology of (Fe,Cr)₇C₃ particulates changes considerably. A dense ceramic layer is formed on the upper surface of (Fe,Cr)₇C₃/Fe surface gradient composite with a volume fraction of 90%. The microhardness of the dense ceramic layer is 1484 HV_0.1, and its relative wear resistance is five times higher than that of the iron matrix.     

Synthesis-microstructure-mechanical properties-wear and corrosion behavior of an Al-Si (12%)—Flyash metal matrix composite

In the present investigation, Aluminium based metal matrix composite containing up to 15% weight percentage of flyash particulates were successfully synthesized using vortex method. The properties like density, hardness, microhardness, ductility and ultimate tensile strength were investigated. The MMC produced was also subjected to corrosion, dry sliding wear and slurry erosive wear test to investigate its behavior under different material wearing conditions. The results of microhardness revealed higher hardness of the matrix material in the immediate vicinity of flyash particle. The addition of flyash particles reduces the density of composite while increasing some of their mechanical properties. The results of wear studies have shown that the resistance to wear increases with increase in percentage of flyash. Corrosion resistance decreases with increase in flyash content. The macrostructural and microstructural characteristics of the MMC were investigated with particular emphasis on the distribution of flyash particles in the matrix. Macrostructural studies have shown near uniform distribution of flyash particles in matrix. Analysis of fractured surface of tensile test specimen is also made which revealed brittle fracture behavior of MMCs.     

Heat treatment and wear characteristics of Al/SiCp composites fabricated by duplex process

This study was undertaken to investigate the effects of alloying elements and heat treatment on the microstructures, wear resistance, and heat resistance of Al–Si–Cu–Mg–(Ni)/SiC_p composites fabricated by a duplex process that consists of squeeze infiltration (1st step) followed by squeeze casting (2nd step). This duplex process produces a homogeneous distribution of SiC_p in Al alloy. The hardness of the composites increased with decrease in SiC_p size, and also with Ni addition in both the as-cast and the as-aged specimens. Compared with 5 and 10 μm SiC_p reinforced Al composites, the aging time to obtain the peak hardness was shortened for 3 μm SiC_p reinforced Al composite, because of higher density dislocations on the periphery of SiC_p in the matrix. However, the Al composite reinforced with 10 μm SiC_p was found to have the lowest wear amount as compared with 3 and 5 μm SiC_p composites. The amount of wear in Al/SiC_p composites decreased with increase of the sliding speed because abrasive wear occurred under low sliding speed and block-type wear debris occurred under high sliding speed.     

Fracture toughness behaviour of a magnesium alloy metal-matrix composite produced by the infiltration technique

Metal-matrix composites comprising short δ-alumina fibres embedded in an Mg 10Al0.4Zn alloy were produced by the squeeze infiltration process, with a fibre volume fraction of 20%. Tensile, hardness and fracture toughness tests were performed at room temperature on both the alloy matrix and the composite in the as-cast as well as in the T6 heat-treated condition. In the as-cast condition it was found that the composite had a markedly increased stiffness, tensile strength and hardness but slightly lower ductility and fracture toughness than the alloy matrix. Whilst a T6 heat treatment improved the mechanical properties of the magnesium alloy matrix, it adversely affected the tensile properties and fracture toughness of the composite.     

Forging of the AA2618/20 vol.% Al2O3p composite: Effects on microstructure and tensile properties

The use of aluminium-based particulate reinforced MMCs for automotive components and aircraft structures have been shown to be highly advantageous over their unreinforced alloys, due to their high specific strength and stiffness and superior wear resistance in a wide temperature range. The aim of this paper was to evaluate the effect of the hot forging process on the microstructure and tensile properties (at room and high temperature) of a MMC based on the aluminium alloys AA2618 reinforced with 20 vol.% of alumina particles (Al₂O_3p). Microstructural analyses of the as-cast and heat-treated composite showed large grain size of the aluminium alloy matrix and a quite non-homogeneous distribution of the reinforcing particles. The forging process led to an evident grain refinement, while it did not lead to significant variations in the size and distribution of the reinforcement particles. Regarding the effect of the forging process on the mechanical properties, it induced a slight increase in hardness, tensile strength, elastic modulus and an evident increase in tensile elongation. SEM analyses of the fracture surfaces of the tensile specimens showed substantially similar morphologies for the as-cast and forged composites, both at room and high temperature. The mechanism of damage was mainly decohesion at the matrix–particle interface.     

The effect of heat treatment on high temperature mechanical properties of microalloyed medium carbon steel

In the present work, high temperature tensile properties and abrasive wear performance of a microalloyed medium carbon steel has been examined. Tensile and abrasive wear testing were carried out on as-received and heat treated specimens. The research has shown that microalloyed medium carbon steel was susceptible to dynamic strain ageing due to interaction of mobile dislocations and solid atoms, such as carbon and/or nitrogen. The interaction between dislocations and solid atoms at 200–300 °C changes the work hardening rate and contributes to dynamic strain ageing. These interactions also increased abrasive wear resistance of the microalloyed medium carbon steel at 300 °C. Therefore, the inference can be drawn that dynamic strain ageing caused an improvement on abrasion resistance.     

Effect of on-axis tensile loading on shear properties of an orthogonal 3D woven SiC/SiC composite

The present study examines in-plane and out-of-plane shear properties of an orthogonal 3D woven SiC fiber/SiC matrix composite. A composite beam with rectangular cross-section was subjected to a small torsional moment, and the torsional rigidities were measured using an optical lever. Based on the Lekhnitskii’s equation (Saint–Venant torsion theory) for a orthotropic material, the in-plane and out-of-plane shear moduli were simultaneously calculated. The estimated in-plane shear modulus agreed with the modulus measured from ±45° off-axis tensile testing. The effect of on-axis (0°/90°) tensile stress on the shear stiffness properties was also investigated by the repeated torsional tests after step-wise tensile loading. Both in-plane and out-of-plane shear moduli decreased by about 50% with increasing the on-axis tensile stress, and it is mainly due to the transverse crack propagation in 90° fiber bundles and matrix cracking in 0° fiber bundles. It was demonstrated that the torsional test is an effective method to estimate out-of-plane shear modulus of ceramic matrix composites, because a thick specimen is not required.     

Effects of lead on annealing properties of cold-drawn copper wire

Cold-drawn copper wires were made by cold-drawing after hot-rolling tough pitch copper rods which had been pre-heated at 673–973 K. The wires were annealed at 293–673 K to study their annealing properties. The effects of pre-heating the hot-rolled tough pitch copper rods, including lead, on the recrystallization behaviour of the cold-drawn copper wires were investigated by tensile tests, electrical resistivity measurements and structural observations. The half-softening temperature of annealed cold-drawn copper wires decreased when the hot-rolled copper rods were pre-heated at 873 K before cold-drawing. This behaviour was attributed to separation of lead dissolved in the copper matrix which is known to lower the half-softening temperature.     

The tensile deformation behavior of Ti–3Al–4.5V–5Mo titanium alloy

The tensile deformation behavior of Ti–3Al–4.5V–5Mo titanium alloy was studied. The results show that there are obvious yield points on true stress–true strain curves of annealing structures, then a stress drop occurs. The curves show linear work-softening after yielding at annealing temperature of 720–780 °C and linear work-hardening at annealing temperature of 800–840 °C. Elastic energy stored in the α-phase is dramatically released after plastic deformation of the β-phase, which leads to the stress drop.     

In situ fabrication of titanium carbide particulates-reinforced iron matrix composites

Titanium carbide (TiC) particulates-reinforced iron matrix composites were prepared by in situ fabrication method combining an infiltration casting with a subsequent heat treatment. The effects of different heat treatment times (0, 1, 6 and 11 h) at 1138 °C on the phase evolution, microstructural features, and properties of the composites were investigated. The as-prepared composites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and microhardness and wear resistance tests. The XRD results showed that the composites consisted of graphite, α-iron and titanium carbide after heat treatment at 1138 °C for 11 h. The SEM observation revealed that the formed TiC particulates were homogenously distributed in the iron matrix. The average microhardness of the composite heat treated at 1138 °C for 6 h increased depending upon the region: 209 HV_0.1 (iron matrix) < 787 HV_0.1 (titanium wire) < 2667 HV_0.1 (composite region). After being heat treated at 1138 °C for 11 h, the composite indicated no considerable change in microhardness value, and the average microhardness of the composite region was about 2354 HV_0.1. The highest microhardness value obtained for the composite region was due to the formation of titanium carbide particulates as reinforcement phase within the iron matrix. Relative wear resistance was determined by a pin-on-disc wear test technique under different loads, and as a result, the composites containing higher volume fraction of hard titanium carbide particulates presented higher wear resistance compared with the unreinforced gray cast iron.     

Reinforcement of Hadfield steel matrix by oriented high‐chromium cast iron bars

To attain a wear‐resistant material compatible with high hardness and high toughness, Hadfield steel matrix was reinforced by oriented high chromium cast iron bars, through inserting high chromium alloys flux‐cored welding wires into Hadfield steel melt at 1500 ± 10 °C. The obtained composites were investigated by XRD, SEM, micro‐hardness, three‐body abrasion wear and impact toughness testers. The results show that the alloy powders inside the flux‐cored welding wires can be melted by the heat capacity of Hadfield steel melt and in situ solidified into high chromium cast iron bar reinforcements tightly embedded in the matrix. The micro‐hardness of reinforcements of the water‐quenched composite is about four times higher than that of the matrix. The impact toughness of the water‐quenched composite is higher than that of the as‐cast composite and lower than that of Hadfield steel, and its fracture mechanism is very complicated and refers to brittle and ductile mixture fracture mode. The excellent impact toughness and better wear resistance of the water‐quenched composite are attributed to combine fully the advantages and avoid the drawbacks of both Hadfield steel and high chromium cast iron. Additionally, in industrial application, the pulverizer plate produced by this composite, has also better wear resistance compared to the reference Hadfield steel pulverizer plate.     

添加Sc、Zr和退火对Al-Si合金铸态力学性能的影响

使用OM、TEM、SEM、显微硬度和室温拉伸等手段研究了Sc和Zr的复合添加对Al-5.5Si合金铸态的组织和性能的影响,以及在不同温度退火后其性能的变化规律。结果表明,Sc、Zr的添加使Al-5.5Si合金的硬度提高了33%、抗拉强度提高了38%、屈服强度提高了52%、延伸率基本上不变。在Al-5.5Si合金中复合添加Sc、Zr使α-Al的平均晶粒尺寸从203 μm减小到130 μm,在α-Al基体中析出大量的Al₃(Sc_1-xZr_x)纳米粒子(10~15 nm),并使共晶Si内的层错或微孪晶的密度显著提高。退火温度对铸态合金的性能有较大的影响：在较低温度(低于160℃)退火时合金的硬度呈上升趋势,而在较高温度(高于280℃)退火时合金的硬度呈显著下降趋势。这些结果与二次析出的纳米Si相密切相关。     

Effect of post-weld heat treatment on the interface microstructure of explosively welded titanium–stainless steel composite

In this study, the interface microstructure evolutions of the explosively welded cp-Titanium/AISI 304 stainless steel composites due to heat treatment are presented. The composites were subjected to heat treatment process at temperature ranges of 650–950 °C in argon atmosphere for 1 h. The investigations were carried out by using optical microscopy (OM), scanning electron microscopy (SEM). The results reveal the presence of reaction layers in the diffusion zone. The compositions of the reaction products were determined by energy dispersive spectroscopy (EDS). Furthermore, the composition–penetration plots for Ti, Fe, Cr and Ni across the interface were obtained from EDS microanalysis. Concentration of discontinuities in the profiles indicating the formation of intermetallics in the diffusion zone that were also detected by X-ray diffraction (XRD) method, on the Ti-side of fractured surfaces. The results show that post-heating of the composite layers in these temperatures causes to form different intermetallic phases at the joint interface. Moreover, post-heating increases the width of interfacial layers of the composite.     

A novel thermal treatment on a Mg–4.2Y–2.3Nd–0.6Zr (WE43) alloy

A double-stage thermal treatment has been adopted on a Mg–Y–Nd–Zr (WE43) alloy, following suggestions of a previous calorimetric investigation. A secondary precipitation is claimed to occur at a temperature as low as 150 °C after a preliminary precipitation at 210 °C, with the effect of enhancing the hardness increase and reducing the annealing times.     

Tensile properties and thermal stability of ZnO-coated aluminum borate whiskers reinforced 2024Al composite

ZnO-coated aluminum borate whiskers reinforced 2024Al composite was fabricated by squeeze casting. Interfacial microstructures and tensile properties of the composite in as-cast and after thermal exposure were investigated. Fracture mechanisms of the composite in as-cast and after thermal exposure were also investigated. The results show that ZnO coating of the whiskers reacts with molten 2024Al and MgAl₂O₄ forms at the interface during squeeze casting. On the one hand, the interfacial reaction between ZnO and 2024Al can improve the wettability of the whiskers by molten 2024Al, increasing the tensile properties of as-cast composite. On the other hand, during thermal exposure, MgAl₂O₄ at the interface can effectively hinder serious interfacial reactions between the whiskers and magnesium in the matrix of 2024Al, improving the thermal stability of the composite at high temperatures.